Memory devices are used to store information in semiconductor devices and systems. The popular dynamic random access memory (DRAM) cell includes a switch and a capacitor. DRAMs are highly integrated and fast memory devices, but they do not retain data when power is cut off.
On the other hand, a nonvolatile memory device is capable of retaining data even after power is cut off. Examples of nonvolatile memory devices include the flash memory, magnetic random access memories (MRAMs), ferroelectric random access memories (FRAMs) and phase-change random access memories (PRAMs). MRAMs store data using variations in the magnetization direction at tunnel junctions. FRAMs store data using polarization characteristics of ferroelectricity. PRAMs store data using resistance variations caused by phase changes of specific materials.
The ideal semiconductor memory includes random accessibility, non-volatile characteristics, increased capacity, increased speed, reduced power consumption, and unlimited reading and writing functions. Resistive Random Access Memory (RRAM) technology has been gradually recognized as having exhibited those semiconductor memory advantages.
RRAM cells are non-volatile memory cells that store information by changes in electric resistance, not by changes in charge capacity. In general, an RRAM cell includes a storage node in which a bottom electrode, a resistive switching layer and a top electrode may be sequentially stacked. The resistance of the resistive switching layer varies according to an applied voltage. An RRAM cell can be in a plurality of states in which the electric resistances are different. Each different state may represent a digital information. The state can be changed by applying a predetermined voltage or current between the electrodes. A state is maintained as long as a predetermined operation is not performed. The manufacturing process and the stability of the RRAM operations are issues needing to be improved for RRAM technology.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.